Prior Art
It has been widely known that the driving stability of a motor vehicle is lost or its steering performance is impaired in some cases, depending on the conditions of the road surface, if the wheels are locked when sharp braking is applied to the motor vehicle. Therefore, it has been in practice to employ an anti-skid control system to control the braking force by decreasing, increasing, or maintaining the pressure of the brake fluid working on the wheel cylinders lest the wheels should be locked at the time of sharp braking, and such a system is also called "anti-lock system". The anti-skid control system as applied to the wheels of a motor vehicle takes the form of such control over the rear wheels or that of such control over the front wheels and the rear wheels, namely, the four wheels. The former prevents the rear wheels from locking and can reduce the braking distance of the motor vehicle. The latter can additionally prevents the front wheels from locking and can therefore maintain the steering performance in a good condition.
In the anti-skid control system, it is designed to control the braking force in such a manner as eventually to maintain the slip factor of the wheels at approximately 20 per cent, namely, to secure the maximum coefficient of friction, by controlling the pressure of the brake fluid in accordance with the rotating acceleration (including deceleration here as in the subsequent part of this Specification) of the wheels in view of the fact that the wheel velocity declines abruptly immediately before the coefficient of friction .mu. attains the maximum value in relation to the wheels as the pressure of the brake fluid applied to the wheel cylinders is increased. With respect to such control performed on the braking force applied to the individual wheels, it is found necessary to perform adequate control over the braking force in accordance with the state of recovery of the wheel velocity on the driving wheels because the driving wheels will be liable to the occurrence of their locking at an early stage if the same control of the braking force as at the time when the power train is disconnected is performed at the time when the power train is connected, for the moment of inertia working on the driving wheels is considerably different between the time when the power train forms a connection between the engine and the driving wheels and the time when the power train does not form any such connection (the difference being related to the presence or absence of the "engine brake" effect), particularly in respect of such control applied to the driving wheels.
As an example of the existing anti-skid control system which performs its control of the braking force in accordance with the state of recovery of the wheel velocity, the system disclosed in the Official Gazette for Patent Laid Open No. 32290-1978 may be cited. Specifically, this cited system is designed to set up a simulated acceleration signal, which is set in advance on the basis of the minimum value of the wheel velocity in the process of the recovery of the wheel velocity on the driving wheels after a pressure reduction of the brake fluid pressure, to prevent an extension of the braking distance by effecting a sharp increase of the brake fluid pressure in case the recovery of the actual wheel velocity is in excess of the velocity indicated by the simulated acceleration signal, assuming in such a case that the power train is disconnected, and to prevent the occurrence of an early-stage lock of the driving wheels by controlling the pressure of the brake fluid for its moderate increase, assuming that the power train is connected.